At present, hexagonal strontium ferrite and barium ferrite are used as the oxide permanent magnet material. To improve magnetic properties, these magnets are often given anisotropy by pressing in a magnetic field. One of magnetic properties is a residual magnetic flux density or remanence Br. Factors largely affecting the remanence Br have the following relationship. It is noted that a saturation magnetization (σs) per unit weight in the following formula is a value intrinsic to a material.Br ∝(saturation magnetization per unit weight)×(density)×(degree of orientation)
Therefore, for the manufacture of anisotropic sintered ferrite magnets having high Br, it is very important to increase the sintered density and the degree of orientation. One common practice employed in the prior art for achieving a high degree of orientation is to mold a slurry having ferrite particles dispersed in water, that is, wet molding. For providing high coercivity, on the other hand, it is necessary to reduce the size of ferrite particles to or below the critical single domain diameter of 1 μm for defining single domains. Such particles have the problem that the degree of orientation generally lowers even when the wet molding technique is used. The probable causes include (1) a likelihood of submicron particles to coalesce, (2) an increase of magnetic coalescence as a result of dividing particles into single domains, (3) a decrease of torque for particles to orient in a magnetic field direction, and (4) an increase of frictional force due to the increased surface area of particles.
For solving this problem, we found that the magnetic coalescence can be reduced by introducing comminution strains into submicron ferrite particles to temporarily reduce the coercivity thereof (see JP-A 6-53064).
We further found that by using an organic solvent such as toluene or xylene instead of water and adding a surfactant such as oleic acid, a degree of magnetic orientation of about 98% at maximum is achievable even with submicron ferrite particles (see also JP-A 6-53064). However, the organic solvent used in this method is detrimental to the human body and the environment. The solvent problem can be solved by a set of large-size recovery and related units, which requires an increased cost.
It is noted that the degree of magnetic orientation used in this specification is a ratio (Ir/Is) of residual magnetization (Ir) to saturation magnetization (Is).
On the other hand, for improving a degree of orientation in the wet magnetic field molding technique using water, it was attempted in the prior art to add a polymeric dispersant as typified by a polycarboxylic acid (or salt) to magnetic particles and allow the dispersant to be adsorbed on the surfaces of magnetic particles whereby the particles are dispersed by virtue of steric hindrance and electrical repulsion, thereby improving a degree of orientation (see JP-A 6-112029). Nevertheless, the degree of orientation and remanence Br thus accomplished are not so high.
Understandably, the problem that the degree of orientation deteriorates as the particle diameter decreases arises not only in the manufacture of ferrite magnets, but also when other particulate oxide magnetic materials such as needle soft magnetic ferrite are oriented in a magnetic field.
We proposed in U.S. Pat. No. 5,951,937 a means of improving the orientation of magnetic particles in a magnetic field during the wet molding step using water by adding a dispersant as typified by gluconic acid or a neutralized salt thereof or a lactone thereof, which means is advantageous from the environmental and economical standpoints. This method is highly effective in improving the degree of orientation, but adversely affects the ability of wet molding in a magnetic field, sometimes leading to an unacceptable decline of productivity.